This invention relates to an improved receiver selecting system and particularly to a system for selecting from a plurality of receivers the one receiver which provides the highest quality signal.
In mobile communication systems, the talk back range of the mobile or portable transceiver is limited because of the output power capabilities. The talk back range can be extended by placing additional receivers in strategic locations, so that during transmission from a portable or mobile unit from any location in the expected coverage area, at least one receiver will receive a signal of sufficient strength to effectively communicate to the dispatcher or main station. In the majority of cases, an acceptable signal will be present at more than one of these satellite receivers.
The audio from each satellite receiver is generally transmitted over telephone lines to a central location where electronic hardware is used to determine which receiver has the best quality audio which is then fed to a common output. The system by which the selection of the best quality signal from a number of inputs is provided, as described above, is referred to herein as the receiver selecting system. This type of system is sometimes known as "receiver voting." To further complicate matters, the signal to noise (S/N) ratio from a particular satellite receiver can change dramatically from the best choice to one that is unacceptable during a signal transmission due to "Rayleigh fading." The fading rate is determined by the channel frequency and the relative motion between the transmitting and receiving antennas. In the 900 MHz band, the fading rate can approach 10 milliseconds.
Systems or arrangements for selecting the best quality signal from a plurality of radio receivers as exemplified by U.S. Pat. No. 4,030,040 of Harbert or U.S. Pat. Nos. 3,403,341 and 3,495,175 of Munch. The signal indicative of the signal quality is compared to a common reference voltage. The reference signal in Harbert is the ramp voltage and in Munch U.S. Pat. No. 3,403,341, it is a fixed reference voltage. In the Munch, U.S. Pat. No. 3,495,175, the common reference voltage is adjusted to follow the highest quality signal where there is a separate comparison circuit including a diode coupled between each of the inputs and a common load wherein the highest quality signal forward biases its diode and back biases the other diodes. The comparison in these prior art systems is done by simultaneously comparing the plurality of signal sources to a common reference voltage to get the highest or lowest level signal. However, if the received signals are of the same or approximately the same quality, these prior art systems can cause multiple gating without some form of priority circuit. Further, this reference voltage would need to be kept precise to prevent false triggering. Also, where the reference voltage is dependent on the forward bias of a diode and back bias the others, the sensitivity is dependent on the voltage offset of the diodes and the characteristics of the diodes. Further, it is desirable to provide a system which inherently ranks the quality of the inputs and/or converts the voltage to a binary number which can be used for further processing.